Method and device for open-loop control of single-phase or multiphase A.C. power controllers

ABSTRACT

In the open-loop control of A.C. power controllers through the phase-angle control of semiconductor valves (4), the chronological sequence of the firing signals (Z) for the semiconductor valves (4) is determined by reference signals (X). When the reference signals (X) fail for one or more periods, this chronological sequence is disturbed and damage to the load (2) to be controlled can result. Therefore, according to the invention, a method is provided whereby a safety firing signal (ZS) is always derived from the preceding firing signals (Z) when the expected reference signal (X) fails. In addition, a device for implementing the method is specified.

BACKGROUND OF THE INVENTION

The present invention relates to a method and a device for open-loopcontrol of single-phase or multiphase A.C. power controllers throughphase-angle control of semiconductor valves.

Alternating current (A.C.) power controllers are used to provide forclosed-loop control of the power supplied to an electrical load in anA.C. system. These power controllers are triggered through thephase-angle control of semiconductor valves. Particularly during theoperation of an A.C. motor, changing operational conditions, such asdifferent loading of the motor during start-up or lag phases when it isturned ON and OFF, require a closed-loop control of the power suppliedto the motor to protect the power system, the motor and the driving gearfrom unnecessary loads.

British Patent 2 084 359 discloses a device for open-loop control of anA.C. power controller for an A.C. motor. This device is supposed toimprove an unfavorable power factor caused, for example, by underloadingof the motor. For this purpose, controllable semiconductor valves, inparticular a triac, a bidirectional triode thyristor, or an antiparallelthyristor circuit, are assigned to the motor for each lag phase. Thesesemiconductor valves enable power to be supplied in dependence upon theprevailing operating conditions of the motor by means of phase-anglecontrol. The power factor is improved with this known device because thephase difference between the current and voltage is detected for eachphase in a forward controlling element and reduced by properlyincreasing the ignition angle, i.e., the period of time that elapsesbetween the current zero crossing and the point of ignition.

In the case of the known device, the instant of time of current zerocrossing, which is determined by measuring the voltage applied acrossthe triac, is used as the time reference for determining the point ofignition. This voltage is fed to a comparator, whose output statescorrespond to the circuit states of this triac. The current zerocrossing corresponds then to an edge of the output signal from thecomparator. From this edge and with the help of a monoflop, a strobepulse is generated for a ramp voltage that is synchronized with the zerocrossing of the supply voltage. The sampled value of the ramp voltage issubtracted from a reference voltage that is input externally by way of apotentiometer and transmitted to the inverting input of a differentialamplifier, whose output voltage is fed, together with the ramp voltageto a further comparator. That second generator generates a primaryfiring signal via a downstream trigger pulse generator, when the rampvoltage exceeds the output voltage of the differential amplifier.Therefore, with this circuit arrangement, the phase shift between themotor current and the motor voltage, and thus the power factor, isstabilized at a value which is specified by the reference voltage set onthe potentiometer.

Practice has shown, however, that there are problems when the instantsof current zero crossings are determined by measuring the voltage dropacross the semiconductor valve, reactions can occur as the result ofinduced voltages, particularly when there are inductive loads. Thesereactions make it more difficult to reliably determine the instants whenthe current goes to zero. Then, as a result of inductive reactionsemanating from a continuously turning rotor, for example, it can happenthat the voltage measured across the semiconductor valve does not reachthe threshold value required to switch over the comparator, so thatdisturbances occur in the course of the firing sequence. In multiphaseA.C. motors, direct-current components can then build up and result inthe motor being subjected to shock or sudden impact loads.

To substantially eliminate these types of disturbances, one must selectthe lowest possible threshold value for the comparator. However, thistype of solution entails increasing sensitivity and in turnsusceptibility to faults caused by system disturbances and inductivevoltage surges in the control of inductive loads.

Therefore, in view of the sensitivity of the measuring device, acompromise must always be made when determining a reference instant forthe time control of the phase angles.

SUMMARY OF THE INVENTION

The present invention addresses the problems giving rise to thiscompromise and specifies a method for open-loop control of single-phaseor multiphase A.C. power controllers, which is substantially insensitiveto external disturbances, while providing a high performancereliability. The present invention also provides a device forimplementing the method.

In a method according to the present invention there is open loopcontrol of single phase or multiphase A.C. power controller throughphase angle control of semiconductor valves. Reference signals aredetected from the current flowing in at least one phase to derivetime-delayed firing signals. A safety firing signal always follows aspecified time interval, that is derived from the time-delayed firingsignals, when no reference signal is detected in the specified timeinterval.

Since the semiconductor valves are even fired when the reference signalthat controls the chronological sequence of the firing signals fails forone or more periods, the thresholds required for generating a referencesignal can be increased. Therefore, it is less likely for thechronological sequence of the firing to be disturbed by unintentionalfirings, and one does not have to put up with the firing beinginterrupted for one or more phases. This is particularly advantageousfor open-loop control of motors, since these measures permit anoperation that is less susceptible to faults.

The time interval between the firing signal and the subsequent safetyfiring signal can thereby be constant and preferably corresponds to anangular distance of about 180°.

A preferred embodiment of the method according to the present inventionprovides for a variable time interval which is dependent on the numberof reference signals that are missing in succession or that come toolate. From one period to another namely, fluctuations in the phaserelation between current and voltage can occur and can cause thesubsequent reference signal to be shifted by more than 180°. This canlead to a continuous firing in the case of firing angles which liewithin the range of these fluctuations, which is not desirable. To avoidthis, a time interval that corresponds to an angle greater than 180°,preferably about 185° is provided for the first safety firing signal.According to a particularly preferred embodiment of the method, toprevent the accepted phase shift from adding up when the referencesignal fails repeatedly during subsequent periods, the time interval toa second safety firing signal is selected to be less than 180°, forexample 175°. With N successive safety firing signals, one must ensurethen that their respective time intervals P₁, P₂, P_(n), . . . P_(n-1),P_(n), P_(N) satisfy the condition ##EQU1## whereby f represents thesystem frequency and F the permitted phase error, which should always beless than 15°, preferably less than 10°.

The reference signal is essentially used thereby to obtain a timereference for the open-loop control. It can be derived fromcharacteristic instants in the temporal current path or voltagewaveshape. To determine a reference signal, it is advantageous to usethe current zero crossings as characteristic instants; they arepreferably derived then from the voltage drop across the semiconductorvalve. In a preferred embodiment of the present invention, thisreference signal is available in the form of a binary signal with twolevels, whose edges are particularly well suited for timing control.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the invention, reference is made to the drawings, inwhich

FIG. 1 illustrates a block diagram of an embodiment of a device forimplementing the method according to the present invention.

FIGS. 2 through 5 show electric signals used to control the firingoperation that are plotted in a diagram over time to clarify the methodaccording to the present invention.

FIG. 6 shows a block diagram of another embodiment of a device forimplementing the method according to the invention.

FIG. 7 is a flow chart illustrating the operation of the embodiment ofFIG. 6.

DETAILED DESCRIPTION

According to FIG. 1, a load, for example a motor 2, is connected via asemiconductor valve 4, for example a triac or a circuit consisting ofseveral semiconductor valves, such as a thyristor circuit arrangement,to the phase of a single-phase alternating system. The semiconductorvalve 4 is provided with a control electrode 5 and is part of a firingdevice 10, which contains an ignition amplifier 12, for example anopto-triac or an ignition transformer, that is required for operatingthe semiconductor valve 4. An optically fired semiconductor valve canalso be provided in place of an electrically fired semiconductor valve.

The open-loop control of the semiconductor valve 4 takes place throughphase-angle or ignition-phase control. To this end, in dependence uponthe phase relation, a firing signal that causes the semiconductor valve4 to be fired, is applied to the control electrode 5.

In the preferred specific embodiment according to the FIG. 1, areference detector 6 is assigned to the semiconductor valve 4. Thisreference detector 6 determines, for example, the current zero crossingof the current I flowing in the electric circuit as a function of thevoltage drop across the semiconductor valve 4. The current zero crossingis used to establish a time reference for the phase-angle control. Acorresponding reference signal X is made available at the output 8 ofthe reference detector 6. For this purpose, the reference detector 6contains, for example, a comparator, which compares the absolute valueof the voltage drop across the semiconductor valve 4 to a definedthreshold value.

Thus, a reference signal X with two levels is applied to the output 8.One level is a state that indicates that a current is flowing in thephase. If this current is smaller than a limiting current value thatcorresponds to the threshold voltage value the second state for thereference signal is provided. By properly selecting a low value for thisthreshold, for example about 10 V, this level essentially correspondsthen to zero current and the reference signal X essentially reproducesthe circuit states of the semiconductor valve 4. The edges between thesetwo circuit states then coincide in time at least roughly with thecurrent zero crossing of the current I flowing through the semiconductorvalve 4.

To establish a time reference for the phase-angle control, one can alsodraw upon another characteristic parameter in the current's time slope,for example the maximum or minimum, or upon a characteristic parameterin the time slope of the voltage.

The output 8 of the reference detector 6 is connected to a forwardcontrolling element 14, which shunts off a time-delayed firing signal Zand holds it ready at a control output 141. This firing signal Z iseither a safety firing signal ZS derived from the preceding firingsignal Z or a firing signal ZX derived from the reference signal X andtime-delayed in accordance with a specified firing angle.

The firing angle is defined by the forward controlling element 14, forexample within the scope of a control program that can be selectedexternally, as well as within the scope of motor parameters that can beinput externally. It is also possible for the firing angle to betime-dependent through a program that runs internally in the forwardcontrolling element 14. Thus, for example, the motor 2 is able to startsmoothly as the result of a time-dependent firing angle, which startingfrom a specified starting value is reduced to a minimal value.

In FIG. 2, the current I flowing through the semiconductor valve and, inFIG. 3, the voltage drop VT existing across the semiconductor valve, areplotted over time. When the current is extinguished at the instant T1,the voltage drop increases and, due to the inductive reaction of therotating motor, at a later instant, T2, attains the threshold value -Lrequired to switch the comparator that follows. The comparatorconfigured in the reference detector 6 then switches, in accordance withFIG. 4, from a signal level 30 to a signal level 32.

These two signal levels 30 and 32 correspond to two logic states, whichindicate whether the voltage drop across the semiconductor valve issmaller or greater in value than the threshold value L. The signallevels 30 and 32 are separated from each other by a first, in theexample of the figure, falling edge 34 and a second, in the example ofthe figure, rising edge 36. In the ideal case, these edges correspond tothe instant the current zero point is reached and the firing instantrespectively.

Starting from the edge 34, the forward controlling element 14 generatesa firing signal Z at the instant T2+DT. This firing signal Z is delayedby the delay time DT and results in the current being re-ignited at theinstant T3. The firing signal Z is generated by the reference signal Xand therefore, in addition, designated as ZX in FIG. 5.

At the instant T4, the current is once more extinguished and the voltagedrop VT across the semiconductor valve rises. However, in the example ofFIG. 3, it does not attain the threshold value +L, which is required togenerate an edge 34 for the reference signal X. This can be the case,for example, when the motor is underloaded for the short term and thevoltages induced in the motor windings when the motor continues to runprevent the voltage drop VT across the semiconductor valve from beinglarge enough to attain the threshold value +L required for switching. Inthis case, at the instant T5, the forward controlling element makesavailable a safety firing signal ZS, which is delayed with respect tothe preceding firing signal Z by an absolute or preset time interval Pand causes the current to be fired at the instant T6. This time intervalP corresponds to an angular distance, which is preferably greater than175° and smaller than 185°, in particular about 180°.

At the instant T7, the current is once again extinguished and, in theexample of FIG. 3, a voltage drop is generated across the semiconductorvalve and, at the instant TS, reaches the threshold value -L required toswitch the comparator. After a time delay DT, a firing signal Z isgenerated, which causes the current to be re-ignited at the instant T9.This firing signal, in turn, is generated by the reference signal X andis therefore likewise designated as ZX in FIG. 5.

A safety firing signal ZS' is likewise drawn with a dotted line in FIG.5. This safety firing signal ZS' would then be applied when no referencesignal X is transmitted within the time interval T5+P by the referencedetector to the forward controlling element.

In the example of FIG. 5, a time interval P₂ for the safety firingsignal ZS' is also drawn in. It corresponds to the time interval P₁between the firing signal ZX and the safety firing signal ZS. Accordingto an advantageous modification, for example, the danger of continuouslyfiring with the same small firing angle differential can be avoided byhaving variable time intervals P₁ and P₂. In this case, it isparticularly advantageous for P₁ to correspond to an angular distance of180°+D and P2 to an angular distance of 180°-D. Here, D preferablyamounts to about 5°, so that generally the relationship

    P.sub.2n+1 ·f·360°180°+D and P2n·f·360°=180°-D

is satisfied, whereby the running index represents the number of safetyfiring signals in series that are not triggered by a reference signal.This guarantees that the phase error is still limited even when safetyfiring signals occur repeatedly in series.

In an advantageous specific embodiment according to FIG. 6, the forwardcontrolling element comprises a control unit 16, which is connected tothe reference detector. The control unit 16 is connected to afree-running counter 22 and reads the counter reading at an instant thatis specified by the reference signal X, for example, by means of itsfalling edge. This value is stored in a register 20 that is connected tothe control unit 16. The counter 22 and the register 20 are connected toa comparator unit 18, which continually compares the reading of counter22 to the reading of register 20 and, when there is conformity, causesthe control unit 16 to apply a firing signal.

In a preferred specific embodiment, the forward controlling element 14comprises a microprocessor, in which three asynchronous processes areprovided. They are illustrated on the basis of the flow charts of FIG.7. In a first process, the system waits for a reference signal X toappear and then, for example, when a falling edge 34 appears, a presetdelay time DT is added to the current reading TI of the free-runningcounter 22 and recorded in the register 20. Moreover, a further processis provided, which reads out the current reading TI of the free-runningcounter 22 when a firing signal Z appears, for example at the risingedge of the firing signal Z, adds a specified value P to it, andlikewise stores the sum in register 20. A further process tests, if thereading TI of the free-running counter 22 is greater or equal to thereading T of register 20. If this is the case, then a firing signal Z isapplied.

The present invention is clarified for single-phase A.C. powercontrollers on the basis of FIGS. 1 through 7. The same considerationsalso apply, however, for each phase of a multiphase A.C. powercontroller.

What is claimed is:
 1. A method for open-loop control of single-phase ormultiphase A.C. power controllers through phase-angle control ofsemiconductor valves comprising the steps of:a) detecting referencesignals from a current flowing in at least one phase; b) derivingtime-delayed firing signals from the detected reference signals; and c)producing a safety firing signal that follows in a specified timeinterval after a preceding time-delayed firing signal when no referencesignal is detected within said specified time interval, wherein saidsafety firing signal is derived from a preceding time-delayed firingsignal.
 2. The method according to claim 1, wherein said step ofproducing further comprises delaying subsequent safety firing signals bya constant time interval, which corresponds to an angular distance ofabout 180°.
 3. The method according to claim 1, wherein the specifiedtime interval is variable and is dependent on the number of referencesignals X that are missing in succession or that are produced outside ofan expected time period.
 4. The method according to claim 3, wherein atime interval for a first safety firing signal corresponds to an angulardistance greater than 180° and less than 190 °.
 5. The method accordingto claim 4, wherein with N successive safety firing signals, thecondition ##EQU2## is satisfied, whereby P₁, P₂, . . . P_(n), P_(N) arerespective time intervals, N is an integer greater than or equal to 1and f is a system frequency.
 6. The method according to claim 1 furthercomprising the steps of:d) providing as a reference signal a signal withtwo logic states whose first state corresponds to a voltage drop acrossthe semiconductor valve which is greater in value than a specifiedthreshold value, and whose second state corresponds to a voltage dropthat is less in value than said specified threshold value; e) deriving asecond instant of time from an edge between these two logic states,which corresponds to an attainment of the current zero point, and thenstoring said second instant of time for use in providing a time-delayedfiring signal; f) deriving a first instant of time from an edge of thefiring signal and then storing the first instant for use in providing asubsequent, timedelayed safety signal; and g) applying a firing signaldetermined by the instant of time last stored.
 7. A method for theopen-loop control of single-phase or multiphase A.C. power controllersthrough the phase-angle Control of semiconductor valves comprising thesteps of:a) detecting a reference signal from a current flowing in atleast one phase; b) deriving time-delayed firing signals from thedetected reference signal: c) producing a safety firing signal thatfollows in a specified time interval after a preceding time-delayedfiring signal when no reference signal is detected within said timeinterval, said safety firing signal being derived from a precedingtime-delayed firing signal, wherein subsequent safety firing signals aredelayed by a constant time interval, which corresponds to an angulardistance of about 180°; d) providing as the reference signal a signalwith two logic states whose first state corresponds to a voltage dropacross the semiconductor valve which is greater in value than aspecified threshold value, and whose second state corresponds to avoltage drop that is less in value than said specified threshold value;e) deriving a second instant of time from an edge between these twologic states, which corresponds to an attainment of the current zeropoint, and then storing said second instant of time for use in providinga time-delayed firing signal; f) deriving a first instant of time froman edge of the firing signal and then storing the first instant for usein providing a subsequent, time-delayed safety signal; and g) applying afiring signal determined by the instant of time last stored.
 8. A methodfor the open-loop control of single-phase or multiphase A.C. powercontrollers through the phase-angle control of semiconductor valvescomprising the steps of:a) detecting reference signals from a currentflowing in at least one phase; b) deriving time,delayed firing signalsfrom the detected reference signals: c) producing a safety firing signalthat follows in a specified time interval after a preceding time-delayedfiring signal when no reference signal is detected within said timeinterval, said safety firing signal being derived from a precedingtime-delayed firing signal, wherein the time interval is variable and isdependent on the number of reference signals X that are missing insuccession or that are produced outside of an expected time period; d)providing as the reference signal a signal with two logic states whosefirst state corresponds to a voltage drop across the semiconductor valvewhich is greater in value than a specified threshold value, and whosesecond state corresponds to a voltage drop that is less in value thansaid specified threshold value; e) deriving a second instant of timefrom an edge between these two logic states, which corresponds to anattainment of the current zero point, and then storing said secondinstant of time for use in providing a time-delayed firing signal; f)deriving a first instant of time from an edge of the firing signal andthen storing the first instant for use in providing a subsequent,timedelayed safety signal; and g) applying a firing signal determined bythe instant of time last stored.
 9. The method according to claim 8,wherein a time interval for a first safety firing signal corresponds toan angular distance greater than 180° and less than 190°.
 10. The methodaccording to claim 9, wherein with N successive Safety firing signals,the condition ##EQU3## is satisfied, whereby P₁, P₂, . . . P_(n), . . .P_(N) are the respective time intervals, N is an integer greater than orequal to 1 and f is the system frequency.
 11. A device for open-loopcontrol of single phase or multiphase A.C. power controllerscomprising:a) a semiconductor valve in series connection between a loadand a phase of the A.C. system, the semiconductor valve having a controlelectrode; b) a reference detector shunting off reference signals fromthe semiconductor valve; and c) a forward controlling element connectedto said reference detector for supplying time-delayed firing signals,wherein the forward controlling element comprises means for deriving asafety firing signal that follows within a preset time interval, that isalways derived in each case from the time delayed firing signals, whenno reference signal is detected within this preset time interval. 12.The device according to claim 11, wherein subsequent safety firingsignals derived from the sequence of firing signals are delayed by aconstant time interval, which corresponds to an angular distance ofabout 180°.
 13. The device according to claim 11, wherein the timeinterval is variable and is dependent on the number of reference signalsX that are missing in succession or that are produced outside ofexpected time period.
 14. The device according to claim 11, wherein atime interval for a first safety firing signal corresponds to an angulardistance greater than 180° and less than 190°.
 15. The device accordingto claim 11, wherein with N successive safety firing signals, thecondition ##EQU4## is satisfied, whereby P₁, P₂, . . . P_(n), . . . ,P_(N) are the respective time intervals, N is an integer greater than orequal to 1 and f is the system frequency.
 16. A device for open-loopcontrol of single phase or multiphase A.C. power controllerscomprising:a) a semiconductor valve in series connection between a loadand a phase of the A.C. system, the semiconductor valve having a controlelectrode; b) a reference detector shunting off reference signals fromthe semiconductor valve; c) a forward controlling element connected tosaid reference detector for supplying time-delayed firing signals,wherein the forward controlling element comprises means for deriving asafety firing signal that follows within a preset time interval, that isalways derived in each case from the time delayed firing signals, whenno reference signal is detected within this preset time interval whereinsaid forward controlling element comprises: d) a flee-running counter;e) a control unit, which reads out the reading of free-running counterat an instant determined by the reference signal; f) a register, whichis loaded by said control unit with a value that results from a sum ofthe counter reading that is read out and a preset delay time; and g) acomparator unit, which compares the reading of the counter to thereading of the register and, in dependence upon the result of thecomparison, causes a firing signal to be applied.
 17. A method foropen-loop control of single-phase or multiphase A.C. power controllersthrough phase-angle control of semiconductor valves comprising the stepsof:a) producing firing signals being used for firing the semiconductorvalves; b) producing reference signals when the current flowing in atleast one phase reaches a predetermined characteristic value, each ofsaid reference signals defining a time instant at which the currentreaches its characteristic value; c) testing whether a reference signalhas been produced within a specified time interval following a precedingfiring signal; and d) deriving as a firing signal a time-delayed firingsignal from the reference signal if a reference signal has been producedwithin said specified time interval or a safety firing signal from apreceding time-delayed firing signal, if a reference signal has not beenproduced within said specified time interval, said safety firing signalfollowing in said specified time interval after said precedingtime-delayed firing signal.
 18. The method according to claim 17,wherein said step of deriving further comprises delaying subsequentsafety firing signals by a constant time interval, which corresponds toan angular distance of about 180°.
 19. The method according to claim 17,wherein the specified time interval is variable and is dependent on thenumber of reference signals X that are missing in succession or that areproduced outside of an expected time period.
 20. The method according toclaim 19, wherein a time interval for a first safety firing signalcorresponds to an angular distance greater than 180° and less than 190°.21. The method according to claim 20, wherein with N successive safetyfiring signals, the condition ##EQU5## is satisfied, whereby P₁, P₂, . .. P_(n), . . . , P_(N) are respective time intervals, N is an integergreater than or equal to 1 and f is a system frequency.
 22. The methodaccording to claim 17, further comprising the steps of:e) providing as areference signal a signal with two logic states whose first statecorresponds to a voltage drop across the semiconductor valve which isgreater in value than a specified threshold value, and whose secondstate corresponds to a voltage drop that is less in value than saidspecified threshold value; f) deriving a second instant of time from anedge between these two logic states, which corresponds to an attainmentof the current zero point, and then storing said second instant of timefor use in providing a time-delayed firing signal; g) deriving a firstinstant of time from an edge of the firing signal and then storing thefirst instant for use in providing a subsequent, time-delayed safetysignal; and h) applying a firing signal determined by the instant oftime last stored.
 23. A device for open-loop control of single phase ormultiphase A.C. power controllers comprising:a) a semiconductor valve inseries connection between a load and a phase of the A.C. system; b)means being coupled to said semiconductor valve for producing firingsignals that are used for firing the semiconductor valve; c) a referencedetector for producing reference signals when the current flowingthrough said semiconductor valve reaches a predetermined characteristicvalue, each of said reference signals defining a time instant at whichthe current reaches its characteristic value; and d) means for testingwhether a reference signal has been produced within a specified timeinterval following a preceding firing signal, wherein e) said means forproducing firing signals derives a time-delayed firing signal from thereference signal if a reference signal has been produced within saidspecified time interval and derives a safety firing signal from apreceding time-delayed firing signal if a reference signal has not beenproduced within said specified time interval, whereby said safety firingsignal follows in said specified time interval after said precedingtime-delayed firing signal.